Nowadays, since CO2 emission regulations are being strengthened in response to increasing concern about environmental problems, weight reduction of an automobile body for increasing fuel efficiency is an issue to be addressed in order to reduce CO2 emission in the automobile industry. In order to solve this issue, there is a growing trend toward reducing the thickness of a high-strength steel sheet used for automobile parts. For example, there is a growing trend toward using a steel sheet having reduced thickness and a TS of 1180 MPa or more.
Here, a high-strength steel sheet which is used for the structural members and reinforcing members of an automobile is required to have excellent formability. In particular, when parts having a complex shape are formed, such a steel sheet is required to be excellent not in terms of a single property such as elongation or hole expansion capability but in terms of plural properties. Moreover, a high-strength steel sheet which is used for automobile parts such as structural members and reinforcing members is required to be excellent in terms of collision-energy-absorbing capability. Increasing yield ratio is effective for increasing collision-energy-absorbing capability, and, by increasing yield ratio, it is possible to efficiently absorb collision energy with a small amount of deformation. Here, the term “yield ratio (YR)” refers to the ratio of yield stress (YS) to tensile strength (TS) and is expressed as YR=YS/TS.
In addition, in the case of a steel sheet having a tensile strength of 1180 MPa or more, there may be a problem in that delayed fracturing (hydrogen embrittlement) occurs due to hydrogen entering from a usage environment. Therefore, a steel sheet having a tensile strength of 1180 MPa or more is required to be excellent in terms of press formability and delayed fracturing resistance.
Conventionally, a dual-phase steel (DP steel) sheet including a ferrite-martensite structure is known as a high-strength steel sheet having both excellent formability and high strength. For example, Patent Literature 1 discloses a technique in which the balance between elongation and stretch flange formability is increased by controlling the distribution state of cementite grains in tempered martensite. In addition, Patent Literature 2 discloses, as a steel sheet excellent in terms of formability and delayed fracturing resistance, a steel sheet in which the distribution state of precipitates in tempered martensite is controlled.
In addition, examples of a steel sheet having both high strength and excellent ductility include a TRIP steel sheet including retained austenite. In the case where such a TRIP steel sheet is subjected to deformation due to forming work at a temperature equal to or higher than the temperature at which martensite transformation starts, it is possible to achieve large elongation due to the transformation of retained austenite into martensite induced by stress.
However, in the case of such a TRIP steel sheet, since the transformation of retained austenite into martensite occurs when punching work is performed, a crack occurs at an interface with ferrite, which results in a disadvantage in that the steel sheet is poor in terms of hole expansion capability.
Therefore, Patent Literature 3 discloses a TRIP steel sheet having increased elongation and stretch flange formability as a result of including, in terms of area ratio, 60% or more of bainitic ferrite and 20% or less of polygonal ferrite. In addition, Patent Literature 4 discloses a TRIP steel sheet excellent in terms of hydrogen embrittlement resistance as a result of controlling the volume fractions of ferrite, bainitic ferrite, and martensite.